The background description provided herein is for the purpose of generally presenting the context of the present invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions.
Optical communications have been widely applied in a long-distance trunk network to a metropolitan area network and an access network due to the characteristics of large capacity, long transmission distance, high transmission speed, economy and the like. Development objectives of an optical fiber communication technology always aim at higher transmission rate, larger capacity and longer transmission distance, so as to continuously increase and improve performance indexes of an optical fiber and a communication technology of the optical fiber. Especially in recent years, with the explosive increase of IP service traffic, a communication network is stepping to the direction of next-generation sustainable development, and construction of an optical fiber infrastructure having a great transmission capacity is a physical foundation of a next-generation network. In order to meet development requirements of an optical fiber communication system, relevant performance indexes of an optical fiber serving as an optical fiber communication network transmission medium also need to be further improved.
Attenuation coefficient of optical fiber is one of the most important performance of the optical fiber, which determines a repeater distance of optical fiber communications to a great extent. As the attenuation coefficient of the optical fiber is smaller, a transmittable distance of an optical signal carried therein is longer. Under the same transmission distance, the attenuation amplitude of the optical signal carried therein is smaller. Reduction of the attenuation coefficient can effectively improve an OSNR (Optical Signal to Noise Ratio) in the optical fiber communications, thus increasing the transmission quality and transmission distance of a system. In a long-distance optical fiber communication, an optical signal is transmitted via a repeater station. If the attenuation coefficient of the optical fiber is smaller, a repeaterless transmission distance of the optical signal will be longer, so the distance between repeater stations can be increased, thereby greatly reducing arrangement of the repeater stations and reducing the operation cost. Therefore, regardless of optimization of a system structure or reduction of the operation cost, reduction of the attenuation coefficient of the optical fiber plays a very important role. On the other hand, with the continuous development of FTTX in recent years, the performance of an original G.652 optical fiber has been difficult to meet requirements of users, and a practical application environment requires an optical fiber to have certain bend resistance performance. So, on the basis of the G.652 optical fiber, a new-generation bend-insensitive single-mode optical fiber, namely a G.657 optical fiber, is developed, including a G.657.A-class optical fiber compatible with the G.652 standard and a G.657.B-class optical fiber not compatible with the G.652 standard. The G.657.A-class optical fiber is well compatible with a G.652.D optical fiber, possesses better bend resistance performance as compared to a common G.652.D optical fiber, and is thus regarded as one of products which most probably replace a conventional G.652 optical fiber. Hence, a new-generation of a single-mode optical fiber compatible with the G.652 standard and with lower attenuation, a relatively large mode field diameter and bend insensitivity becomes a research focus in the field of communication optical fibers.
In a manufacturing process of an optical fiber preforming bar, the optical fiber attenuation can be reduced by generally adopting the following methods. For example, higher-purity raw materials are adopted, so as to improve the sealing performance of a production environment and equipment, and to reduce the introduction probability of external impurities. For example, in Chinese Patent Application No. CN201110178833.3, a method of improving the air tightness of a deposition process of an optical fiber preforming bar is adopted, so as to reduce introduction of the external impurities. Or, a manufacturing process of a preforming bar with a larger outer diameter is adopted, and the overall attenuation of an optical fiber is reduced by means of a dilution effect of a large-size preforming bar. In addition, in a manufacturing process of the optical fiber, a coating process for a bare optical fiber surface coating is also an important factor which influences the attenuation performance of the optical fiber. However, no matter from the perspective of cost and process control in theoretical optical fiber preparation or from the perspective of cost and process control in actual optical fiber preparation, reducing doping of the optical fiber and optimizing the section of the optical fiber are simple and effective methods for reducing the optical fiber attenuation. Generally speaking, as the concentration of a doping material is lower, losses caused by Rayleigh scattering will be smaller. In a traditional single-mode optical fiber, in order to ensure a total reflection in the optical fiber, a sufficient refractive index difference value between a core layer and an inner cladding must be ensured, the relative refractive index of the core layer being much larger than that of the inner cladding of the optical fiber. In order to ensure the design, doping of Ge or co-doping of Ge/F must be performed in the core layer to a large extent. In a traditional optical fiber section design, laser energy is distributed in an optical fiber section in a Gaussian distribution form, about 70% of the laser energy is propagated in a heavily-doped core layer part. That is, high-energy density laser transmission propagation is concentrated in a high-concentration doped core layer with a relatively large Rayleigh coefficient. If a section with energy in non-Gaussian distribution is designed by means of reasonable optical section design to reduce energy losses in the high-concentration doped core layer, the attenuation performance of the optical fiber can be significantly reduced.
However, in these conventional G.657 optical fiber section designing and manufacturing methods, Ge and F are co-doped in the core layer to a large extent. In order to obtain optimal macrobend performance, the relative refractive index of the core layer is larger than 0.35% generally, that is, Ge is doped in the core layer to a large extent, and therefore larger Rayleigh scattering will be caused so as to increase the attenuation of the optical fiber.
Chinese Patent Application No. CN201310394404 provides a design of an ultra low attenuation optical fiber. The ultra low attenuation optical fiber is designed with a pure silica outer cladding. However, the ultra low attenuation optical fiber adopts a typical step section structure instead of a trench cladding for optimizing bending of the optical fiber, and a core layer of the ultra low attenuation optical fiber is not doped with Ge. Therefore, viscosity mismatch during preparation of a preforming bar may be caused, and it can be found out that the attenuation and bending levels are relatively poor.
Attenuation of a quartz optical fiber in a range of 600 nm to 1600 nm mainly comes from Rayleigh scattering, and the attenuation αR caused by the Rayleigh scattering can be calculated according to the following formula:
      α    R    =                    1                  λ          4                    ⁢                        ∫          0                      +            ∞                          ⁢                              R            ⁡                          (              r              )                                ⁢                      P            ⁡                          (              r              )                                ⁢                      rdr            /                                          ∫                0                                  +                  ∞                                            ⁢                                                P                  ⁡                                      (                    r                    )                                                  ⁢                rdr                                                          =                  R                  λ          4                    +      B      
In the formula, λ is wavelength (μm), and R is Rayleigh scattering coefficient (dB/km/μm4); P is light intensity; and when the Rayleigh scattering coefficient is confirmed, B is a corresponding constant. Therefore, as long as the Rayleigh scattering coefficient R is determined, the attenuation αR (dB/km) caused by the Rayleigh scattering can be obtained. The Rayleigh scattering is caused by both density fluctuation and concentration fluctuation. Therefore, the Rayleigh scattering coefficient R can be expressed as:R=Rd+Rc 
In the above formula, Rd and Rc are respectively representative of variations of the Rayleigh scattering coefficient caused by the density fluctuation and the concentration fluctuation. Where Rc is a concentration fluctuation factor, which is mainly influenced by the doping concentration of an optical fiber glass part. In theory, as fewer Ge and F or other elements are doped, Rc is smaller. That is why certain foreign enterprises adopt pure silicon cores at present to achieve the ultralow attenuation performance.
However, it is important to note that the Rayleigh scattering coefficient further includes another parameter Rd. Rd is associated with a fictive temperature TF of glass, and varies along with the structure variation and temperature variation of the glass. The fictive temperature TF of the glass is a physical parameter representative of a glass structure, and is defined as a temperature corresponding to a certain state of equilibrium achieved from rapid cooling of the glass at a certain temperature T′ to no more adjustment of the glass structure at room temperature. When T′>TF (softening temperature of glass), the glass structure is easy to adjust due to low viscosity of the glass, and therefore the glass is in a state of equilibrium at each moment, so TF=T′; when T′<Tg (transition temperature of glass), the glass structure is difficult to adjust due to high viscosity of the glass, and the structural adjustment of the glass lags behind the temperature variation, so TF>T′; and when Tg<T′<TF (softening temperature of glass), time needed for making the glass tend to be equilibrated is shorter, and the time is associated with components of the glass and a cooling speed specifically, so TF>T′ or TF<T′.
During design with a pure silicon core, in order to ensure the total reflection of the optical fiber, an F-doped inner cladding with a lower refractive index must be used for matching, so as to ensure that a sufficient refractive index difference is maintained between the core layer and the inner cladding. So, the viscosity of the core layer part of the pure silicon core is relatively high, and meanwhile, the viscosity of the inner cladding part doped with a great number of F is relatively low, so as to cause viscosity mismatch of an optical fiber structure. Thus, the virtual temperature of the optical fiber with a pure silicon core structure rapidly rises to make Rd of the optical fiber increase. So, not only advantages brought by Rc reduction are canceled, but also the attenuation of the optical fiber may be reversely abnormal.
U.S. Pat. No. 6,917,740 discloses a pure silicon core optical fiber improved in performance by utilizing viscosity variation. A great number of F and Cl are doped in the core layer, Rayleigh coefficients of the optical fiber are reduced by utilizing contributions of F and Cl doping to the viscosity of the core layer, the section design for the optical fiber is not involved herein, and Ge is not doped in the core layer.
PCT Application No. PCT/US2010/022533 discloses a design of an optical fiber. In order to obtain a lower Rayleigh coefficient, pure silicon core design is adopted, germanium and fluorine are not co-doped in a core layer, and fluorine-doped silica is adopted to serve as an outer cladding. As for design of the pure silicon core, complicated viscosity matching must be performed inside the optical fiber, and an extremely low speed is required in a drawing process to avoid attenuation increase caused by defects inside the optical fiber due to high-speed drawing, so the manufacturing process is extremely complicated.
A common ultra low attenuation single-mode optical fiber is designed with an outer cladding fully doped with F. From the perspective of fiber optics, the design is relatively simple, and as long as a refractive index difference value between the outer cladding and the corer layer is ensured, requirements for total reflection of the optical fiber can be met. However, there are mainly two factors for limiting the manufacturing cost of the ultra low attenuation optical fiber at present: first, the size of a preforming bar purely doped with F is small, and a drawing process is complicated; and second, the manufacturing cost of an optical fiber purely doped with F is very high due to an F doping process. The price of an F-doped casing is 5 to 8 times that of a pure silica casing by means of preliminary estimate according to a current market price. On the basis of calculation according to a preliminary relationship that the cost of an F-doped material is 6 times that of a pure silica material, if a reasonable design process is adopted to appropriately reduce the thickness of an F-doped layer, the manufacturing cost of the optical fiber will be significantly reduced. If an F-doped material is merely used from 30 μm (diameter of the optical fiber) to 80 μm and common pure silica is used from 80 μm to 125 μm, this design reduces the material cost by 40% as compared to traditional design of an ultra low attenuation optical fiber with a fully doped F material. If an F-doped material is used from 30 μm to 60 μm and common pure silica is used from 60 μm to 125 μm, the material cost is reduced by 65%.
The above analysis shows a feasibility of performing process design on an ultra low attenuation optical fiber by using a non-pure silicon core and a partially doped fluorine cladding. However, due to influence of the foregoing two limitation factors, how to control optical parameters of an optical fiber under the design is a facing challenge.
If pure silica not doped with fluorine is used as an outer cladding material, 3 problems will be caused.
First, inhibition of fundamental mode cutoff: a refractive index difference value between the outer cladding material and the core layer material is too small, and leakage of an optical fiber fundamental mode will be caused, so as to influence the attenuation of the optical fiber. So, reasonable optical fiber section design must be adopted to inhibit leakage of the fundamental mode between the outer cladding and the core layer of the ultra low attenuation optical fiber with a non-F-doped outer cladding material.
Second, consideration of viscosity matching: if any viscosity optimization design is not made in the outer cladding material, the viscosity is mismatched with the viscosity gradient of the inner cladding and the core layer.
Third, consideration of optical section matching: if pure silica glass is used as the outer cladding material, the doping concentration of each part will be limited when viscosity matching design is considered. In order to ensure that optical parameters of the optical fiber meet parameter requirements of a G.652 or G.654 optical fiber, that is, to ensure that the MFD, dispersion and bending performance of the optical fiber meet standard requirements, optical section design must be taken into consideration. This requires taking the optical design of the optical fiber into comprehensive consideration during the viscosity design, thus increasing the difficulty of process implementation.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.